1. Field of the Invention
The invention relates to solid oxide electrolyte, fuel cell generator modules, modules disposed in a common pressure vessel for use with a variety of auxiliary components in a power generation system operating in either an atmospheric or pressurized mode.
2. Description of the Prior Art
Fuel cell based, electrical generator apparatus utilizing solid oxide electrolyte fuel cells (xe2x80x9cSOFCxe2x80x9d) arranged within a housing and surrounded by insulation are well known, and taught, for example, by U.S. Pat. No. 4,395,468 (Isenberg) and xe2x80x9cSolid Oxide Fuel Cellxe2x80x9d, Westinghouse Electric Corporation, October, 1992. The tubular type fuel cells can comprise an open or closed ended, axially elongated, self-supporting ceramic tube air electrode material, completely covered by thin film ceramic, solid electrolyte material. The electrolyte layer is covered by cermet fuel electrode material, except for a thin, axially elongated, interconnection material. Flat plate type fuel cells can comprise a flat array of electrolyte and interconnect walls, where electrolyte walls contain thin, flat layers of cathode and anode materials sandwiching an electrolyte. The xe2x80x9ccorrugatedxe2x80x9d plate type fuel cells can comprise a triangular or corrugated honeycomb array of active anode, cathode, electrolyte and interconnect materials. Other fuel cells not having a solid electrolyte, such molten carbonate fuel cells are also well known, and can be utilized in the article and method of this invention.
Development studies of SOFC power plant systems have indicated the desirability of pressurized operations in many instances. This would permit operation with a coal gasifier as the fuel supply and/or use of a gas turbine generator as a bottoming cycle. Integration is commercially possible because of the closely matched thermodynamic conditions of the SOFC module output exhaust flow and the gas turbine inlet flow.
Conventional combustor in a gas turbine system typically exhibits high nitrogen oxides (NOx) emissions, combustion driven oscillations and instabilities, excessive noise and low efficiencies. Although significant advances have been made to mitigate these problems, it has proved difficult to design a practical, ultra-low NOx, high-turn-down ratio combustor due to poor flame stability characteristics. The combination of all the above factors results in a pressurized SOFC generator module design being suitable as a replacement of conventional gas turbine combustor and applicable to more efficient combined cycle power plants required to meet increasingly stringent emission targets.
A variety of fuel cell uses in power plant systems are described in the literature. U.S. patent application Ser. No. 09/784,610, filed on Feb. 15, 2001, Holmes et al.) discloses a low-cost atmospheric SOFC power generation system, which provides a simpler, significantly less expensive oxidant/air feed tube support system and an improved power lead design. The inexpensive design minimizes cooling requirements, external ducting to auxiliaries such as air blowers, air pre-heaters and recuperators and provides better utilization of interior insulation. In U.S. Pat. No. 3,972,731 (Bloomfield et al.), a pressurized fuel cell power plant is described. There, air is compressed by compressor apparatus, such as a compressor and turbine which are operably connected, which is powered by waste energy produced by the power plant in the form of a hot pressurized gaseous medium, such as fuel cell exhaust gases. These exhaust gases are delivered into the turbine, which drives the compressor for compressing air delivered to the fuel cells. In U.S. Pat. No. 5,413,879 (Domeracki et al.) a SOFC is also integrated into a gas turbine system. There, pre-heated, compressed air is supplied to a SOFC along with fuel, to produce electric power and a hot gas, which gas is further heated by combustion of unreacted fuel and oxygen remaining in the hot gas. This higher temperature gas is directed to a topping combustor that is supplied with a second stream of fuel, to produce a still further heated gas that is then expanded in a turbine. Gillett et al., in U.S. Pat. No. 5,750,278 taught a self-cooling, mono-container fuel cell generator design with integral cooling ducts that could be used for both atmospheric and pressurized operation. The atmospheric and pressurized design, however, would be housed in different containment vessels. In U.S. Pat. Specification No. 5,573,867 (Zafred et al.) a pressurized modular design SOFC generator was housed in a transportable, low center of gravity, horizontally disposed pressure vessel which also allowed purge gas passage between the inside of the pressure vessel and the outside of the SOFC modules. That design contained six separate gas entrance/exit lines just on one end of the container contributing to the substantial cost of pressurized systems over atmospheric systems which usually have less complicated external containment design.
Fuel cell pressurization, while advantageous in system performance, presents several practical difficulties to the SOFC generator designer, two of which are: (1) The pressure boundary must be able to withstand pressures up to 20 atmospheres. The pressure boundary of existing generators operating at one atmosphere pressure is the outside SOFC generator wall, which typically operates at temperatures between 600xc2x0 C. and 800xc2x0 C. Construction of a pressure boundary to operate at 20 atmospheres and 800xc2x0 C. is difficult and expensive, therefore, a pressure boundary with reduced wall temperature is required; (2) Because fuel and air are brought together within the SOFC generator, much more care must be taken to avoid the potential of an unstable condition during startup and operation of a pressurized SOFC. This is only a limited concern at one atmosphere. For atmospheric operation, the expected explosive overpressure would be about 115 psi (792 kPA) which existing designs can accommodate by mechanical strength alone. However, the expected explosive overpressure at 20 atmospheres is about 2315 psi (15950 kPA). A protective containment system to prevent the accumulation of an explosive gas mixture is required; (3) the pressurized containment design is usually much more complicated and expensive than that of atmospheric systems. What is needed is a new packaging approach in the overall configuration of a SOFC power generating plant by maintaining a standardized generator module which could be easily reconfigured for inlet and exhaust duct routings in such a way that it can be coupled to a recuperator/duct burner module, to operate as an atmospheric unit, or to a gas turbine, to operate in a pressurized mode.
In view of this, the main objects of this invention are to: (1) provide a standard stack configuration for both atmospheric and pressurized SOFC Modules, (2) provide a common tank design for both systems, (3) reduce assembly time of the generator stack by reducing the number of installed parts, (4) reduce part inventories, (5) improve functionality of the system, (6) improve generator serviceability/maintainability issues, by providing simple means for stack insertion/extraction within the tank, (7) increase the availability of the fuel cell generator, (8) improve the overall efficiency and performance of the power generation system, and (9) ultimately offer a cost-effective solution to the pressing demand for compact, standard, low cost SOFC systems.
The above needs and objects are met by providing a fuel cell generator apparatus characterized by containing at least one fuel cell assembly module comprising at least two side by side subassemblies each containing a plurality of fuel cells, each fuel cell having electrolyte between an oxidant electrode and a fuel electrode; where the subassemblies are each fueled at their base by a common fuel feed injector nozzle attached to a fuel feed pre-reformer which is connected to integral fuel distribution manifolds; a module housing capable of withstanding temperatures over 600 C. enclosing the fuel cell assembly module; an axially-elongated thin wall vessel surrounding the module housing, the vessel having two ends; a purge gas space between the module housing and the pressure vessel; at least one fuel gas feed inlet through the vessel and connecting to the fuel feed injector nozzles; common gaseous oxidant-purge gas feed inlet; exhaust gas outlet through the vessel connecting to a combusted gas exit plenum through a semi-flexible duct; insulation contacting the inside of the vessel within at least part of the purge gas space; where the vessel is adapted to be used for either atmospheric gaseous feed of pressurized gaseous feed. The fuel cells will generally operate at temperatures usually over 650xc2x0 C. and up to about 1100xc2x0 C. The module housing and the fuel cells can operate in the xe2x80x9cpressurizedxe2x80x9d mode, that is over at least about 2 atmospheres, or about 28.5 psi (pounds per square inchxe2x80x94196.4 kPA), preferably at about 2 to 10 atmospheres. During operations, the entire purge area is flooded by the gaseous oxidant which passes around the module housing to provide a uniform temperature distribution and eliminate the need for complicated integral cooling ducts. Preferably the vessel surrounding the module housing is cylindrical.
The invention also resides in a method of operating a fuel cell generator apparatus characterized by the steps of: (1) passing a common gaseous oxidant-purge feed gas and a fuel gas feed through inlets and into at least one fuel cell assembly module, each module comprising at least two side by side subassemblies, each containing a plurality of fuel cells, each fuel cell having electrolyte between an oxidant electrode and a fuel electrode, where the modules are each enclosed by a module housing capable of withstanding temperatures over 600xc2x0 C.; where the module housings are surrounded by an axially-elongated vessel having two ends, such that there is a purge gas space between the module housings and the vessel; (2) passing the common gaseous oxidant-purge gas through the pressure vessel to circulate within the purge gas space, where the gas dilutes any unreacted fuel gas flow from the module; and (3) passing exhaust gas and circulated purge gas and any unreacted fuel gas out of the pressure vessel, where the vessel is adapted to be used for wither atmospheric gaseous feed or pressurized gaseous feed.
The generator apparatus will also have associated with it and will be working in cooperation with well known auxiliaries, such as controls; an oxygen or air pre-heater; a fuel gas compressor; a fuel desulfurizer; an oxygen or air compressor which may be operably connected to a power turbine coupled to an electric generator; a purge gas compressor, which may be the same as the air compressor; a source of fuel gas and purge gas; heat exchangers; a heat recovery unit to recover heat from the hot fuel cell exhaust gases; and a topping combustor, to provide an electrical power generation system. This type power system, in the pressurized mode, could be, for example, part of an integrated, coal gasification/fuel cell-steam turbine combination power plant, featuring a plurality of coal gasifiers and fuel cell generator arrays or power blocks with associated DC/AC conversion switchgear or it could also be part of a natural gas fired combustion turbine system or the like.